Experts warn more research is needed to find out how to reduce the number of babies born early.

Prematurity is the second most common cause of death for children aged five.

An analysis of 39 developed countries, suggests numbers could be cut by measures such as stopping multiple IVF pregnancies - but only by 5%.

But experts writing in the Lancet say the reasons for many early births remain unknown and much more research is needed.

Each year 15 million babies are born before 37 weeks of pregnancy, and rates are rising almost everywhere.

But there is limited understanding as to why this is happening or what could be done.

An estimated 1.1 million premature babies die each year.

Most are born just a few weeks early in developing countries, where they die from a lack of simple care.

But experts believe developed countries can also cut rates.

'Shockingly small'

Child health experts from organisations including the World Health Organization, Save the Children and the London School of Hygiene and Tropical Medicine looked at what could be done in the 39 most developed countries if five recognised measures were implemented.

These interventions are stopping smoking, promoting single pregnancies in IVF treatments, reducing planned Caesarean sections - which are often carried out before due dates - unless there is a medical reason by 80%, providing progesterone supplements to women with high-risk pregnancies and cervical stitches for women with a "weak" cervix that could mean a baby does not go to term.

If all these were implemented, the researchers suggest premature birth could be prevented for 58,000 babies.

Issuing their call in advance of World Prematurity Day on 17 November, the experts say the reductions - which could be achieved by 2015, would vary from 8% in the US to much smaller reductions in most European countries, and only 2% in the UK.

Dr Joy Lawn, of Save the Children, who is part of the Born Too Soon initiative that seeks to cut prematurity rates, said: ""Our analysis shows that the current potential for preterm birth prevention is shockingly small.

"Our hope is that the proposed target of a 5% relative reduction in preterm births in high income countries will motivate immediate programme action, and the 95% knowledge gap will motivate immediate, strategic research.

"Research should also focus on preterm birth causes and solutions in low income countries where preterm birth rates are highest and the underlying causes may be much simpler to address."

Writing in the Lancet, Jane Norman and Andrew Shennan, of Tommy's Centre for Maternal and Fetal Health, at the University of Edinburgh, said: "Until considerable strides have been made in our understanding of how, why and when preterm births occur, and the effects that this has on both mother and baby, preterm births will remain a major public health problem, from which no country in the world is immune."